My current sensor has been in 46 hours. It doesn't give any readings, just the three dashes. Every time I check my BG with a finger stick, it says calibration failed. Please test again later. I have had sensors take 30 hours to start giving readings after several reports of failure to calibrate. Usually they start working around 18 - 20 hours after insertion and three or four attempts to calibrate. My last one was really good and started on second try. Unfortunately the transmitter battery died in 24 hours but i was able to restart it after I replace the battery and it started the second time too around 14 hours. It also gave me excellent readings for next five days, which for me only happens about a third of the time. At what point should you give up on a sensor and figure it just won't work.